1. Field of the Invention
The present invention relates generally to the field of difference amplifiers. More particularly, the present invention relates to a difference amplifier apparatus using operational amplifiers powered by single polarity power supplies, wherein the difference amplifier apparatus has a reduced common-mode response, even when the common-mode signal applied to the input of the difference amplifier is much larger than the differential signal applied to the input of the difference amplifier and much larger than the supply voltage to the amplifier.
2. Discussion of the Prior Art
Difference amplifiers are designed to measure the difference between two voltages applied to the respective inputs of the difference amplifier and to generate an output voltage which is proportional to the difference between the input voltages. The input signal which represents the difference between the two voltages applied at the respective inputs to the difference amplifier is called the differential mode signal. While to being responsive to the differential mode signal, an ideal difference amplifier has no response when the voltage on each respective input terminal is the same, even when this voltage is relatively large. That is, a difference amplifier should have no response when there is no difference between the input voltages. A signal applied to the input of the difference amplifier when both voltages at the respective inputs to the difference amplifier are the same, is called the common-mode signal.
A prior art implementation of a difference amplifier is shown in FIG. 1. An op-amp 12 is used which has infinite open loop gain for most practical purposes. Whenever the term op-amp or operational amplifier is used hereinafter the amplifier meant is of this type having essentially infinite open loop gain. The most important case, in the choice of components, for the circuit illustrated in FIG. 1 is the case where the ratio of the resistors 28 and 30 connected to the non-inverting input of op-amp 12 is identical to the ratio of the resistors 24 and 26 connected to the inverting input. In this case, the voltage at the output of the difference amplifier is the ratio of the resistance of resistor 26 to the resistance of resistor 24 multiplied by the difference between the voltages at the input.
One problem with prior art implementations of difference amplifiers such as illustrated in FIG. 1 occurs when there is a large common-mode voltage applied to the input terminals. Due to performance limitations of the components which are used in the op-amp, the non-inverting input of the amplifier may be driven to a voltage that is beyond its allowable common-mode operating range (either positive or negative depending on the design of the particular amplifier). At voltages greater than the common-mode operating range of the amplifier, the difference amplifier ceases to function property. A "break-through" or "feed-through" voltage may be generated that will feed through the feedback resistors connected to the inverting input of the difference amplifier and show up at the output of the difference amplifier. This feed through voltage will result in an error in response to differential-mode signals between the input terminals, as well as adversely affecting other circuits which may be connected to the output of the difference amplifier.
Another problem with the difference amplifier shown in FIG. 1 occurs when the operational amplifier is powered by a single polarity power supply. In this case, the output of the differential amplifier cannot be driven to an opposite polarity voltage and, in fact, due to limitations of the components within the op-amp, in actual practice, the output of the amplifier cannot be driven fully to the ground (or zero) voltage. Therefore, the difference amplifier cannot respond to bipolar input voltages. That is, since the output of the difference amplifier cannot be driven negative when a positive polarity power supply is used, the difference amplifier cannot reflect a change in the difference between the voltages applied at the non-inverting and inverting inputs when the voltage applied to the inverting input becomes larger and more negative than the voltage applied to the non-inverting input.
A third problem with the prior art implementation of a difference amplifier occurs when the common-mode voltage applied to both inputs of the difference amplifier is not a DC voltage, but is a voltage which has a significant amount of high frequency energy. A high-frequency common-mode voltage may interfere with the desired response of the difference amplifier to the differential-mode signal.
Therefore, an object of the present invention is to provide a difference amplifier apparatus in which there is a reduced effect on the output of the difference amplifier due to a common-mode signal that is much larger than the differential voltage of interest.
Another object of the invention is to provide a difference amplifier apparatus that operates from a single polarity power supply and that can respond to bipolar input voltages.
Still another object of the invention is to provide a difference amplifier apparatus capable of rejecting common-mode voltages that are higher than the power supply voltage.
A further object of the invention is to provide a difference amplifier apparatus in which different gains can can be provided by using a combination of positive feedback and negative feedback.
Yet another object of the present invention is to provide a difference amplifier apparatus incorporating the present invention which is manufactured using monolithic integrated circuit technology and is reliable, inexpensive, and easy to manufacture.